Many materials, particularly military supplies, are packaged under controlled relative humidity (R.H.) conditions in order to protect them from deterioration. One of the prior art methods for monitoring R.H. in containers was based on viewing a chemically impregnated card through an air-tight window held in a button type plug package which was sealed in an orifice in the container wall. The chemically impregnated card had treated marked spots thereon which responded by changing color when the relative humidity was within a specific range. The button type package humidity card indicator has been described in U.S. Pat. No. 2,716,338. The aforedescribed prior art device had problems because it was often difficult for an observer to read the cards and distinguish with assurance whether the color was blue, blue-pink, or pink. Since the color reading was subjective from person to person, it was very difficult to get an accurate reading. In addition, people who are color-blind would have additional problems in making a proper determination of the exact reading. Another problem with the prior art chemical card type indicator was that, after extended usage, the chemicals tended to leach out of the cards thereby making the readings indistinguishable. Another problem with the go-no go type of card indicator was that it was not possible to determine how far the atmosphere in the container was above or below a determined critical level of R.H. Such information is desirable during humidity control conditioning of the container after loading, and also when the container is exposed to temperature changes. A further problem with the prior art card device was that it was not possible to monitor humidity in a container with reasonable precision when using disc indicators. Experience has shown that R.H. readings taken with color change indicators may be off the actual value by as much as 20%.